Kenyan police killed at least 11 people in a crackdown on protests as anger at the reelection of President Uhuru Kenyatta erupted in the western city of Kisumu and slums ringing the capital, officials and witnesses said on Saturday.

The bodies of nine young men shot dead overnight in Nairobi's Mathare slum had been brought to the city morgue, a security official told Reuters. The men were killed during police anti-looting operations, the official added.

Separately, a young girl in Mathare was killed by police firing "sporadic shots", a witness said. The run-down neighbourhood is loyal to 72-year-old opposition leader Raila Odinga, whose party rejected Tuesday's vote as a "charade".

A Reuters reporter in Kisumu, centre of post-election ethnic violence a decade ago in which 1,200 people died nationwide, said tear gas and live rounds were fired. One man had been killed, a government official said.

The unrest erupted moments after Kenya's election commission announced late on Friday that Kenyatta, 55, had secured a second five-year term in office, despite opposition allegations that the tally was a fraud.

Interior Minister Fred Matiang'i said the trouble was localised and blamed it on "criminal elements" rather than legitimate political protest.

Odinga's NASA coalition provided no evidence for its rejection of the result. Kenya's main monitoring group, ELOG, said on Saturday its tally matched the official outcome, undermining NASA's allegations of fraud.

In addition to the deaths, Kisumu's main hospital was treating four people for gunshot wounds and six who had been beaten by Kenyan police, its records showed.

One man, 28-year-old Moses Oduor, was inside his home in the impoverished district of Obunga when police conducting house-to-house raids dragged him out of his bedroom and beat him with clubs.

"He was not out fighting them. He was rescued by my sister who lives next to him. She came outside screaming at the police, asking why they are beating people," his brother, Charles Ochieng said, speaking on behalf of a dazed Oduor.

More shooting was heard outside the hospital on Saturday morning. In Nairobi, armed police units backed by water cannon moved through the rubble-strewn streets of Kibera, another pro-Odinga slum.